Fuse assembly for oil-filled transformers

ABSTRACT

A fuse assembly including a body member with condenser caps attached to each end thereof. A fuse wire is connected to the condenser caps and extends through one opening in a block of arcextinguishing material which is contained within the body member. Another opening in the block is positioned parallel to the opening containing the fuse wire. With moderate fault currents, arcing as the fuse wire melts is confined within the opening containing the fuse wire. With large fault currents, the arcextinguishing material between the two openings ruptures, thus enlarging the arcing passageway to reduce the arc energy. Hot gases and vapors generated during arcing melt solder which seals openings in the condenser caps, thus the gases and vapors may pass through the unsealed openings to relieve pressure within the fuse assembly.

United States Patent [191 Braunstein 11] 3,827,011 [451 July 30,1974

[ FUSE ASSEMBLY FOR OIL-FILLED TRANSFORMERS [75] Inventor:

[22] Filed: Mar. 8, 1973 [21] Appl. No.: 339,243

Harry R. Braunstein, Athens, Ga.

Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney,Agent, or Firm-J. R. Hanway [57] ABSTRACT A fuse assembly including abody member with con denser caps attached to each end thereof. A fusewire is connected to the condenser caps and extends through one openingin a block of arc-extinguishing material which is contained within thebody member. Another opening in the block is positioned parallel to theopening containing the fuse wire. With moderate fault currents, arcingas the fuse wire melts is confined within the opening containing thefuse wire. With large fault currents, the arc-extinguishing materialbetween the two openings ruptures, thus enlarging the arcing passagewayto reduce the arc energy. Hot gases and vapors generated during arcingmelt solder which seals openings in the condenser caps, thus the gasesand vapors may pass through the unsealed openings to relieve pressurewithin the fuse assembly.

1 Claim, 4 Drawing Figures FUSE ASSEMBLY FOR OIL-FILLED TRANSFORMERSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates, in general, to electrical inductive apparatus and, morespecifically, to fuse assemblies containing an arc-extinguishingmaterial.

2. Description Of The Prior Art Fuse links and current-limiting fusesare used in power distribution transformers either separately or in aserial combination thereof to protect the electrical distribution systemand to prevent explosion of the distribution transformer. Normally, thefuses are connected in series with the primary of high-voltage windingof the transformer. When a major fault occurs within the transformer,the fuse interrupts the extremely high fault current which is developed.

Fuses or protective devices for such applications necessarily offer acompromise between current interrupting capability, refusingfeasibility, coolant contamination levels, construction costs, and otherfactors. Generally, it is desirable to protect a transformereconomically with a fuse capable of interrupting the maximum amount ofprimary fault current which could flow through the transformer.

Fuse links capable of interrupting 3,500 amperes are economicallyavailable. However, since the possibility of a fault current of 8,000amperes at 7,200 volts exists with modern power distributiontransformers, prior art protective links are inadequate.Current-limiting fuses rated at 8,000 amperes are relatively expensive.Therefore, it is desirable, and it is an object of this invention, toprovide an economic fuse assembly which will interrupt at least 8,000amperes of fault current.

Fault current magnitudes are dependent on several factors, including thetype of fault. Protective devices for use in interrupting fault currentsmust be capable of proper operation regardless of the magnitude of thefault current, provided the current does not exceed the maximumoperating current of the protective device. However, it is desirablethat the are produced while interrupting the current dissipate an amountof energy which is within a workable range. Without enough are energy,the arc-extinguishing material does not function properly. With too muchare energy, the fuse assembly may explode. Therefore, it is alsodesirable, and it is another object of this invention, to provide a fuseassembly which is constructed to maintain the arc energy within suitablelimits.

Most arc-extinguishing materials are considered contaminants totransformer cooling and dielectric oil. Therefore, during normaloperation, the arcextinguishing material must be sealed from thetransformer oil. However, the fuse assembly must be vented when a faultcurrent is interrupted to prevent explosion of the fuse assembly.Therefore, it is also desirable, and it is a further object of thisinvention, to provide a fuse assembly which is properly sealed whennormal current is flowing therethrough and which is vented properly whena fault current is being interrupted.

SUMMARY OF THE INVENTION There is disclosed herein a new and useful fuseassembly for use in oil-filled transformers. The fuse assembly includesan insulating body member which is sealed at each end with acondenser'cap. A block of arc-extinguishing material is disposed withinan opening in the body member; Two longitudinal openings extend throughthe block. A fuse wire extends through one of the openings and isattached to the condenser caps. With moderate size fault currents, theare which occurs when the fuse wire melts is contained within the oneopening. Due to the small size of the opening, the are energy issufficient to cause the block to evolve arcextinguishing gas or vapor.When the fault current is large, the region of the block located betweenthe openings therein ruptures and effectively increases the size of thearc path. This decreases the arc voltage to limit the amount of arcenergy and to prevent explosion of the fuse assembly. Openings in thecondenser caps are plugged with solder to seal the arc-extinguishingmaterial from the oil. When an arc develops, the hot gases and vaporsmelt the solder to allow the gases and vapors to flow through theopenings and into the trans former tank.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of thisinvention will become more apparent when considered in view of thefollowing detailed description and drawing, in which:

FIG. 1 is a cutaway view of an oil-filled transformer containing a fuseassembly constructed according to this invention;

FIG. 2 is a view, partly in section, of a fuse assembly constructedaccording to this invention;

FIG. 3 is a sectional view taken along the line IIIIII of FIG. 2; and

FIG. 4 is a sectional view taken along the line IVIV of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the followingdescription, similar reference characters refer to similar elements ormembers in all the figures of the drawing.

Referring now to the drawing, and to FIG. 1 in particular, there isshown an oil-filled transformer 10 constructed according to thisinvention. The transformer tank 12 supports the high-voltage bushing 14,the lowvoltage bushings 16, and the core and coil assembly which is notillustrated. The bracket 18 supports the clips 20 and 22 which areelectrically connected to the bushing 14 and to the transformer winding,respectively. The fuse assembly 24 is held by the clips 20 and 22 belowthe oil level 26. The fuse assembly 24 interrupts the current flowingtherethrough when the current exceeds a predetermined value.

FIG. 2 illustrates details of the fuse assembly 24. A substantiallytubular body member 26 is constructed of an insulating material such asglass reinforced epoxy or polyester resins. The body member 26 includesan opening 28 extending between the ends of the body member 26 to whichare attached the condenser caps 30 and 32.

The condenser cap 30 is constructed similar to the condenser cap 32.Condenser cap 32 is thimble-shaped and is constructed of an electricalconducting material such as brass or copper. A suitable condensermaterial having a high specific heat is placed within the condenser caps30 and 32, such as the copper wool 34 in condenser cap 32.

The condenser cap 32 is attached to the end of the body member 26 byforming the end of the condenser I cap 32 in a groove 36 extendingaround the outside of the body member 26. A flat or tubular gasket 38 isdisposed between the condenser cap 32 and the body member 26 to providean oil-tight seal. A fluoro-silicon rubber gasket material is especiallysuitable due to its excellent thermal and oil-resistant properties.

A slug or block 40 of arc-extinguishing material is disposed within thebody member 26. The arcextinguishing material may be a water vaporevolving material such as boric acid (H 80 The block 40 is cylindricallyshaped and extends substantially between the condenser caps 30 and 32.

As illustrated in FIGS. 2 and 3, two longitudinal openings 44 and 46extend through the block 40. A suitably rated fuse wire 48 extendsthrough the opening 46 and is attached to the condenser caps 30 and 32by brazing, welding, or by another suitable process.

The openings 44 and 46 in the block 40 cooperate to provide a desirablefuse action with different magnitudes of overload current. When theoverload current flowing through the fuse wire 48 is relatively low, the

block 40 functions as a thermal insulator positioned around the fusewire 48. This helps melt the fuse wire 48 to interrupt the overloadcurrent. Since the arc voltage increases as the diameter of thepassageway through which the arc travel decreases, the small diameter ofthe opening 46 provides a relatively high are voltage This is desirablesince at least a minimum amount of are energy must be developed topermit proper action of the arc-extinguishing material in block 40.Since are energy is a function of the arc voltage and the overloadcurrent, with low overload currents a high are voltage is desirable.When the overload current is relatively high but still within theoperating limits of the fuse assembly 24, the arc voltage should berelatively low to reduce the are. energy generated. If the arc energy istoo high, the fuse assembly 24 fails to function as desired and mayexplode.

The opening 44 is suitably dimensioned and positionedto lower the arcvoltage when the overload current is relatively high. Opening 44 isseparated from opening 46 by the portion 50 of the block 40. The portion50 has a mass small enough to be ruptured by the are energy developed bya large overload current. This effectively increases the size of thearcing passageway and, therefore, lowers the arc voltage. Thus, when theoverload current is large and tends to develop too much arc energy, thesize of the arcing passageway is increased to keep the arcing energywithin workable limits. in a fuse assembly 24 having a boric acid block40 and an operating limit of 8,000 amperes at 7,200 volts, suitablediameters for the openings 44 and 46 are 0.125 inch. A suitable spacingbetween centers of the openings 44 and 46 is 0.25 inch.

it is necessary that the fuse assembly 24 be sealed to prevent the boricacid block 40 from contaminating the transformer oil. However, somepressure release means must be included in the fuse assembly 24 topermit the escape of high-pressure gases and vapors generated primarilyby the reaction of the block 40 within the arc. Condenser caps 30 and 32contain openings 52 therein which are sealed with tin-lead solder 54 orby another suitable thermoplastic material. The solder 54 has a meltingpoint which allows the solder 54 to remain hard and seal the openings 52when the transformer temperatui'e is within normal and moderatelyoverloaded ranges. The melting point of the solder 54 is low enough topermit the solder to melt when heated by the hot gases generated byarcing within the fuse assembly 24. Other sealing means may also beused, such as a diaphragm placed over the openings 52.

When the fuse assembly 24 interrupts an overload current, the hot vaporsand gases flow into the condenser caps 30 and 32. As is best illustratedwith reference to condenser cap 32, the hot vapor flows through thecopper wool 34 which, due to its high specific heat and lowertemperature, causes water in the vapor to condense thereon, thusreducing the pressure of the vapor after it flows through the copperwool 34. The hot vapor quickly melts the solder 54 in the openings 52and allows the vavapor to escape from the fuse assembly 24, thuspreventing the explosion of the fuse assembly 24.

FIG. 4 illustrates the position of the openings 52 in the end of thecondenser cap 32. The size and position of the openings 52 is importantto achieve the proper melting characteristics although openingarrangements other than the one illustrated are within the contemplationof this invention. If the openings 52 are too small, insufficientexhaust area will cause the pressure to increase within the fuseassembly 24 and rupture the body member 26. Openings too large aredifficult to solder properly and the melting characteristics of thesolder 54 therein cannot be controlled sufficiently because of thenon-uniformity of the conduction of heat from the solder by thecondenser cap material. An opening diameter of A inch has been foundsatisfactory for use in fuse assemblies rated for 8,000 amperes at 7,200volts.

Present protective links for distribution transformers are capable ofinterrupting a 3,500-ampere fault overload current. To achieve8,000-ampere capacity, current limiting fuses have been requiredaccording to the prior art. With the fuse assembly 24 taught by thisinvention, a protective device capable of interrupting 8,000 amperes canbe constructed with the economy herebefore realizable only withprotective links. Thus, current limiting fuse performance can beobtained with protective link economy.

Since numerous changes may be made in the abovedescribed apparatus andsince different embodiments of the invention may be made withoutdeparting from the spirit thereof, it is intended that all of the mattercontained in the foregoing description, or shown in the accompanyingdrawing, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. A fuse assembly suitable for serial connection in the primary circuitof an oil-filled transformer, comprising:

a tubular body member having first and second ends with an openingextending between said ends;

a first cap sealingly attached to the first end of said body member by afluoro-silicon rubber gasket;

a second cap sealingly attached to the second end of said body member bya fluoro-silicon rubber gasket;

said first and second caps containing copper wool and a plurality ofopenings plugged by tin-lead solder which melts and opens said openingswhen the current through the fuse assembly is greater than firstpredetermined value;

a block of boric acid material disposed within said body member openingand extending substantially 6 between said first and second caps, saidblock conallows the portion of said block separating said tammg firstand Second p g them"; and openings to rupture when the current throughsaid a fuse wire connected to said first and second caps d h h l h fuseassembly is greater than a second predeterfi fj mgt mug Onyt 6 OpenmgmSal mined value, said second predetermined value said first and SecondOpenings in Said block being being within the operating limits of thefuse assemsubstantially parallel to each other and spaced from eachother a predetermined amount which

1. A fuse assembly suitable for serial connection in the primary circuitof an oil-filled transformer, comprising: a tubular body member havingfirst and second ends with an opening extending between said ends; afirst cap sealingly attached to the first end of said body member by afluoro-silicon rubber gasket; a second cap sealingly attached to thesecond end of said body member by a fluoro-silicon rubber gasket; saidfirst and second caps containing copper wool and a plurality of openingsplugged by tin-lead solder which melts and opens said openings when thecurrent through the fuse assembly is greater than a first predeterminedvalue; a block of boric acid material disposed within said body memberopening and extending substantially between said first and second caps,said block containing first and second openings therein; and a fuse wireconnected to said first and second caps and extending through only thefirst opening in said block; said first and second openings in saidblock being substantially parallel to each other and spaced from eachother a predetermined amount which allows the portion of said blockseparating said openings to rupture when the current through said fuseassembly is greater than a second predetermined value, said secondpredetermined value being within the operating limits of the fuseassembly.